The present invention generally relates to the field of sinusoidal phase estimation and more particularly to the field of phase estimation for absolute position sensing purposes. Most particularly the present invention relates to phase estimation for absolute angular position sensing.
As is well known, position sensing devices generally fall into two categories; analog, in which a measured signal (for example, a voltage or current) is related to the position of the object being observed in a known way, and digital, in which the output signals from one or more sensing elements are combined and processed to provide a numerical representation of the position of the object. Position sensing devices that provide a numerical representation are often called encoders because the position of the object is “coded” as a number.
Position sensors are also categorized as linear or rotary, which indicates whether they are designed to sense linear or rotary position changes of the object being observed and further can be “incremental” or “absolute” position sensors. Incremental sensors or encoders measure changes in position only and thus require some sort of memory to provide the base point from which the measured changes are evaluated. The typical output of an incremental encoder's sensor head is a simple binary train wherein each pulse increments a counter by one least significant bit. Absolute encoders or position sensors, on the other hand, produce a signal that, at any instant, is related to the object's position relative to an a priori designated zero point.
Encoders and position sensors generally comprise a position indication generator, two or more sensors which serve as the signal source(s) for processing, and an electronic signal processing system. In many systems the position indication generator, when combined with the two or more sensors produces substantially sinusoidal signals, wherein the phase(s) of the sinusoid(s) are related to the position of the object of interest.
If these signals were perfect, relatively simple processing could be successfully applied. However, in many encoder systems the signals are not perfect, so advanced, self-adapting processors are advantageous.
In many previous encoder systems, in which the position indication generator and sensors produce sinusoidal signals, the multiple detectors are closely spaced, often being directly adjacent to each other. Additionally, these detectors are often large when compared to the period of the sinusoid they are sensing. Often, these encoders typically have been designed to sample the sinusoidal signal at 90 degree intervals and in fact have detectors that integrate the signal over substantially a full quarter cycle. Examples of such encoders are taught, for example, by Mitchell et al. in U.S. Pat. No. 5,646,730.
There are several beneficial features inherent in the design of these encoders. Because the multiple detectors are closely adjacent to each other they share many environmental and electrical conditions. For a light-based encoder, for example, the optical power falling on the detectors is relatively uniform. Similarly, if the detectors are part of a custom made linear array, as if often the case, the opto-electrical properties of all the detectors are virtually identical and their output signals pass through virtually identical, on-chip electronic elements (viz., pre-amplifiers). Additionally, detectors that are large, as a fraction of a period, inherently integrate over that fraction of a period, smoothing out noise and eliminating the effects of higher harmonics of the base sinusoid. Finally, it is typical in encoders of this sort of design that the detector array, instead of having just enough detectors to sense one cycle of the sinusoid, have enough detectors to sample some number of adjacent cycles of the sinusoid, improving the signal-to-noise ratio of the output signals and enhancing the averaging out of any position indication generator errors.
Although the above encoders generate high-quality and accurate output signals, they are nonetheless subject to certain kinds of errors that can degrade accuracy. Thus, certain phase estimation methods have been designed to work with the signals generated by these common, previous encoders. For example, see Remillard et al., U.S. Pat. No. 6,897,435, which describes a self-calibrating phase estimation method particularly well suited for processing encoder signals that use the so-called 4-bin process, such as the encoder described in Mitchell (op. cit.).